Chromium is a toxic metal that is regulated in the U.S. by the Clean Air Act (CAA, 1990) and the Resource Conservation and Recovery Act (RCRA, 1986). The Resource Conservation and Recovery Act, which regulates hazardous waste incinerators and boilers and industrial furnaces using hazardous waste fuels, uses risk assessment arguments to limit human exposure. Allowable ground level concentrations, to which the maximum exposed individual is subjected, are determined based upon risk specific doses for four carcinogenic metals and reference air concentrations for eight non-carcinogenic metals. Only the hexavalent form of chromium, Cr(VI), is considered to be a potent carcinogen, and its risk specific dose is correspondingly very low (0.00083 .mu.5g/m.sup.3). Data on chromium speciation in combustion exhaust gases are not readily available, and it is not clear that Cr(VI) is the dominant form, or even a significant portion, of total Cr in incinerator exhausts. However, for the purposes of conforming to the Resource Conservation and Recovery Act, all chromium is assumed to be Cr(VI) unless site specific speciation is performed, which is difficult and expensive. Based upon this assumption, chromium emissions are often a major contributor to the health risk assessment conducted at waste incineration facilities (Bailiff and Kelly, 1990).
Many fossil fuels and incineratable wastes contain chromium constituents. Chromium, like other metallic elements, cannot be destroyed during combustion or incineration processes, although high temperature environments will induce metal transformations. These physical and chemical transformations may exacerbate their harmful effects, since many of the metal species react with other constituents and readily vaporize within combustion environments. Prior experimental work has demonstrated that chromium species introduced into a combustion environment react with other constituents and partition into hexavalent (toxic) and non-hexavalent (relatively non-toxic) species in the exhaust gas. It has been found that adding chlorine increases the fraction of hexavalent chromium, presumably due to the formation of CrCl.sub.6.
Chromium occurs in oxidation states ranging from divalent (II) to hexavalent (VI) (Goyer, 1991). However, only trivalent (III) and hexavalent (VI) states are commonly found in the environment (Seigneur and Constantinou, 1995). Of these two, Cr(III) is the more abundant form, while Cr(VI) compounds are more important industrially. Anthropogenic chromium in ambient air originates from fossil fuel combustion, waste incineration, and industrial sources such as ferrochrome processing, ore refining, and chemical, refractory, and cement production. Cr(III) oxide, Cr.sub.2 O.sub.3, is present in high temperature refractories, and has been shown to promote sulfur capture by calcium products in hot combustion flue gases (Slaughter et al., 1987). Anthropogenic sources account for 60-70% of the atmospheric chromium emissions, with natural sources accounting for the remaining 30-40% (Seigneur and Constantinou, 1995). Typical ambient concentrations of total chromium range from less than 0.0001 .mu.g/m.sup.3 in rural areas to 0.03 .mu.g/m.sup.3 in industrial cities (Goyer, 1991).
At ambient conditions, the vapor pressures of chromium species are negligible; only condensed phases are present. As a result, chromium atmospheric chemistry is associated with solid particles and aqueous droplets. Seigneur and Constantinou (1995) have reviewed the solution chemistry for chromium and developed a kinetic mechanism to describe the conversion of Cr(III) to Cr (VI) and Cr(VI) to Cr(III). They concluded that typical atmospheric conditions favor the reduction of Cr(VI) to Cr(III) species through reactions with trivalent arsenic [As(III)], divalent iron [Fe(II)], vanadium, or sulfur dioxide, although slow oxidation of Cr(III) to Cr(VI) is also possible under some extreme conditions through reactions with manganese.
There is no evidence that Cr(III) is converted to Cr(VI) in biological systems. However, Cr(VI), being a strong oxidizing agent, readily crosses cell membranes where it is reduced to Cr(III) (Trinchon and Feldman, 1989; Goyer, 1991). It has been speculated that the biological effects of Cr(VI) are associated with its biological mobility, reduction to Cr(III), and the formation of intracellular Cr(III) macromolecules (Goyer, 1991). Chromium does not bioaccumulate in the body with the exception of the lungs (Trinchon and Feldman, 1989). Exposure to Cr(VI) is most commonly associated with cancer of the respiratory system, although other adverse health effects including skin ulcers, allergic dermatitis and nasal perforations have been identified (Trinchon and Feldman, 1989; Goyer, 1991).
Thus, Cr(VI), but not Cr(III), has been determined to pose a significant human health hazard. The ensuing health risk depends upon exposure to Cr(VI), but not necessarily upon exposure to total chromium. Since conversion of Cr(III) to Cr(VI) is unlikely through atmospheric or biological mechanisms (Seigneur and Constantinouu, 1995; Goyer, 1991), the major route for human exposure is likely through direct (anthropogenic) releases of Cr(VI) into the environment. It is, therefore, important to determine what portion of total chromium emissions from combustion sources might consist of Cr(VI) and how this depends on combustion conditions, other fuel constituents, and the valence state of chromium entering the system.
It is difficult to measure chromium partitioning in gases for two reasons. First, the dominant chromium species predicted by equilibrium [Cr.sub.2 O.sub.3 (s), cf. FIG. 1] is a condensed species which is difficult to digest for subsequent analysis. Second, care must be taken to ensure that Cr(VI) is not reduced during the sampling and analysis process. This is usually accomplished by keeping the sample in contact with an alkaline environment at all times. The converse problem of oxidizing chromium to Cr(VI) species is not an issue at room temperatures (Seigneur and Constantinou, 1995).
Current models describing chromium partitioning in combustion systems are based on chemical equilibrium. It is important to note, however, that equilibrium may not be achieved in a particular system because of kinetic rate or mixing limitations. In addition, any equilibrium prediction is only as good as the thermochemical information available. Previous studies which included equilibrium predictions or chromium partitioning (Barton et al., 1990; Linak and Wendt, 1993) had access to thermodynamic properties of only a limited number of chromium species, and predicted trends of increased volatility and increased Cr(VI) concentrations caused by large quantities of chlorine.
To date, there has been no way to measure the trace quantities of hexavalent chromium in combustion air streams to determine the exact concentrations of hexavalent chromium and then to convert the hexavalent chromium to trivalent chromium.